Active line scan sensors presently utilized in the prior art make imagery of an object by directing a beam of light generated, for example, by a laser, to the object and sensing the reflected energy. The beam of light is made to scan the object periodically, similar to a flying spot scanner. Resolution of the sensor is determined by the size of the illumination spot on the object, the spot being scanned such that each resolution element on the object is illuminated one at a time. The reflection from each resolution element may be individually recorded on film or other media. The active line scan sensors described are primarily operated from an aircraft, the object being imaged being the ground and targets thereon.
The beam of light, which may be generated from a laser, scans the ground in a direction perpendicular to the path of the aircraft. For each sweep of the sensor, a narrow line equal to the width of the resolution of the sensor is scanned on each side of the aircraft. The motion of the aircraft allows successive adjacent lines to be scanned, the sensor viewing each resolution element as it is illuminated by the laser beam. Part of the optical reflected energy is detected by photodetectors in the sensor, the resulting electrical video signals being amplified, processed and used to control an optical film writing system, such as a strip map.
The prior art systems using optical techniques to measure the distance from a transmitter to a reference plane, such as described in U.S. Pat. Nos. 3,602,594; 3,446,971; 3,619,058; and 3,649,123, although providing reflectivity and distance information, do not provide imagery information, visual or otherwise, including the height, depth and/or slope of objects located at the reference plane. Further, in order to ensure accurate distance measurements with high resolution capability, the scanning laser utilized should be capable of generating light pulses of very precise frequency and phase stability, the pulse train preferably comprising relatively high frequency (harmonic) components.